1. Field of the Invention
The field of the invention is that of nanotubes or nanofibers that may be of the carbon, silicon or boron type or made of any other alloy based on at least one of these components (for example SiC) and possibly containing nitrogen (SiN, BN, SiCN). Typically, these nanotubes or nanofibers have diameters ranging from a few nanometers to a few hundred nanometers over several microns in height and constitute field-emission supertips, being characterized by very high electric field amplification factors.
They are particularly beneficial in field emission devices and especially in the fabrication of field-emission cathodes used as cold electron sources, which have many applications (electron tubes, ion motors, electron microscopy, nanolithography, flat display devices, etc.).
2. Description of Related Art
Conventionally, nanotubes or nanofibers are produced by growing them from catalyst spots very small in size. The use of nanotubes or nanofibers in field-emission devices generally requires the location of each nanotube or nanofiber to be controlled. To do this, the conventional method consists in making submicron (preferably of the order of 100 nm) apertures in a resin and then in depositing the catalyst as a thin film (with a thickness of less than about 10 nm).
After the step of dissolving the resin, catalyst spots having a diameter equivalent to the diameter of the apertures in the resin are then obtained. The application of this method to the fabrication of field-effect cathodes is illustrated in FIG. 1. A field-effect cathode generally comprises the following stack, illustrated in FIG. 1a: a substrate 1, a conducting layer 2, a diffusion barrier 3 (to prevent the catalyst from diffusing into the substrate), an insulator 4 and a conducting film corresponding to the electron extraction grid 5.
A second step consists in making apertures of the order of 100 nm in a resin 6 that is deposited on the stack illustrated in FIG. 1a, then in anisotropically etching the grid and isotropically etching the insulation, by chemical means (FIG. 1b).
The third step then consists in evaporating a catalyst 7 from a source S so as to deposit a catalyst spot in a self-aligned manner with respect to the aperture in the extraction grid.
This process is very suitable if the evaporation source S, which is of small extent (of the order of 1 cm), the aperture in the grid and the aperture in the insulation are aligned (FIG. 1c, right-hand feature). When this is not the case (FIG. 1c, left-hand feature), the catalyst spot is not centered in the cathode and consequently the nanotube or nanofiber grown from this catalyst spot cannot also be aligned.
After having carried out the lift-off operation in order to remove the resin 6 and the excess catalyst 7 (FIG. 1d), the process of growing the nanotubes 8 or nanofibers is carried out (FIG. 1e).
Nanotube or Nanofiber Growth Step
The preparation methods are the following: electrical discharge, pyrolysis, physical methods such as laser ablation and chemical methods such as CVD (chemical vapor deposition) or PECVD (plasma-enhanced CVD).
The method that seems best suited to the field-emission cathode application is the PECVD method, which is assisted by DC plasma, RF (radiofrequency) plasma or microwave plasma. This method allows nanotubes or nanofibers to be obtained that are oriented perpendicular to the substrate.
The nanotubes or nanofibers shown in all the figures of the patent have been drawn schematically.
For example in the case of carbon nanotubes (FIG. 1f), the catalyst particle has an elongate shape and a thin carbon film encapsulates this particle. In addition, unlike nanofibers, nanotubes are hollow.
The diameter of the nanotubes/nanofibers is smaller than the diameter of the catalyst spot, since the size of a catalyst aggregate is smaller than that of the spot and, after growth, the catalyst particle generally assumes an elongate shape.
It is thus apparent that the cathode on the right is operational, whereas that on the left will generate a short circuit. The case illustrated in FIG. 1 corresponds to an extreme case, but is should be noted that a slight misalignment of a nanotube or nanofiber relative to the hole (of the order of 100 nm) in the grid causes incorrect operation of the cathode (C. Xie et al., JVST B18, 1833 (2000)).